PROJECT SUMMARY The proposed research will test related hypotheses that thalamus plays a heretofore neglected and critical role in cortical processing. In particular, many thalamic nuclei, that together comprise the majority of thalamic volume and that were previously mysterious in function, we now suggest are critically involved in information flow and functional, dynamic binding between cortical areas via cortico-thalamo-cortical pathways. We propose to study these pathways using the mouse visual system as the model involving in vitro slice preparations and in vivo behaving preparations. It appears that, in many and perhaps all cases, cortical areas are connected by both direct and these transthalamic pathways, and we wish to understand why: What is different in the information passed by each pathway? Why does one pass through thalamus with the possibility of being blocked there? Is there nonlinear summation in the target cortical area when both pathways are active, and could this be involved in dynamic linking of areas to subserve various cognitive tasks, such as attention? To begin developing answers to these questions, we propose to probe basic circuit properties of these pathways to better understand the role of the transthalamic pathways in higher cognitive functioning.